The present invention relates to an improvement in magnetic recording medium, specially suitable for metal thin film type magnetic recording medium for video tape recording. Ferromagnetic thin film type magnetic recording medium is typically manufactured by forming on a polyester film ferromagnetic thin film such as Fe, Co, Ni or their alloys, by means of vacuum deposition, ion plating, spattering, electrolytic plating, etc. is advantageous in that its recording density can be drastically increased in comparison with the conventional coating of a magnetic film type recording medium. Such magnetic recording medium must have good surface characteristic suitable for the recording. Since the ferromagnetic thin film is quite thin, i.e. having a small thickness of 100 .ANG.-3000 .ANG., the surface characteristic of it reflects those characteristics of the substrate plastic film. On the other hand, it has been found that a magnetic recording medium to be used in high density recording should have a substrate surface having a slight roughness (peak to valley-bottom level difference) of protrusions and indeed a high density distribution of minute unevenness is considered good. As a substrate or film base suitable for the above-mentioned conditions, plastic substrates have been made by forming extremely minute protrusions induced by residue of the catalyst used for polyester polymerization on the surfaces, or by forming a wave-shaped or granular pattern on the surface of the polyester film by forming a cross-linked high molecular weight thin film mainly consisting of denatured silicone resin, epoxy resin or urethane on the surface of the polyester film during a stretching process. Magnetic recording media made by forming a ferromagnetic thin film on the above-mentioned films have advantage that they have a satisfactory still life when encountering friction with video recorder magnetic heads, but the conventional magnetic recording media with a ferromagnetic thin film have the shortcoming that when they are used below 30.degree. C. and at 80%-90% relative humidity (hereinafter referred to as RH) i.e., high moisture conditions, they are liable to make a squealing upon running around a head cylinder part.
The present invention provides a magnetic recording medium having good surface characteristics that is not liable to produce such objectionable squealing.
Ordinary polyester film available in the market includes a low molecular weight polymerized composition which is produced as a by-product during polymerization of the polyester resin, i.e., an oligomer, at a rate of about 1%. In general, a polyester film has a smaller distribution of the oligomer component at its surface because in a heat-set treatment in the final process of making the film, the oligomer on the surface appears to have sublimated from the surfaces, thereby making the surfaces very smooth after the heat set treatment. When such a polyester film with a very smoothly finished surface is exposed to a high temperature, oligomers inside the film diffuse out to the surface forming minute crystal-type depositions on the polyester film surfaces. That is, ordinary polyester film available in the market hardly has any oligomer protrusions of noticeable size which the present invention utilizes to advantage. However, the polyester film is subjected to heat or radiated energies from metal vapor, ion vapor, or a vaporizing source during the processing of forming ferromagnetic thin film by vacuum deposition, ion plating, spattering, etc. for forming the ferromagnetic thin film on the polyester film. As a consequence oligomer protrusions are formed on the surface of the polyester film during the process of forming the ferromagnetic thin film thereon. As a result, the polyester film base when coated with a ferromagnetic thin film is provided with oligomer protrusions beneath the ferromagnetic film. When ordinary polyester film available in the market is used as the substrate, the density of the oligomer protrusions on the substrate surface is only about 100/mm.sup.2 or smaller.